Field of the Invention
The invention is related to decoupling between transmit and receive coils using a transmit array in a magnetic resonance apparatus. It employs decoupling between RF transmit field and the magnetic field of receiver coil by reducing the total current induced in a receive coil by transmit coils. It can be used to achieve concurrent RF excitation and magnetic resonance imaging (MRI) signal reception so that magnetic resonance imaging based on the spin characteristics during excitation, and the concept of zero echo time can be realized. It can also be used in continuous wave MR spectroscopy, continuous wave MRI and MRI with ultra-short echo time (UTE), as well as real time field/impedance monitoring.
Description of the Prior Art
In MRI, both transmit and receive coils are tuned at the resonant frequency. However, coupling from transmit coil to the receive coil causes artifacts to occur in the image. In order to avoid those artifacts, decoupling between transmit and receive units is carried out by decoupling diodes in single or back to back convention where a DC voltage turns the diode(s) on and shifts the resonant frequency in order to avoid signal coupled in the receive coil during transmission. (Halise Irak, MS Thesis, Bilkent University, NOVEL RF COIL TECHNOLOGIES FOR MRI. June 2006) Another point of interest in isolation of transmit and receive units is that spins resonate at the excitation frequency and there is a few orders of magnitude difference in MRI signal and RF excitation signal voltage levels. Isolation between the MRI signal and RF signal is done by time-interleaved separation in most of the conventional MRI methods. In this classical approach however, magnetization characteristics during excitation are ignored. There are applications which use information about spin dynamics in presence of B1 excitation field. In continuous wave NMR, for example, concurrent transmission and reception is used and RF bridges are utilized for isolation between MRI signal and RF signal which increases RF power requirements resulting in excessive heating of the sample. (Schneider, E., Prost, R. W. and Glover, G. H. (1993), Pulsed magnetization transfer versus continuous wave irradiation for tissue contrast enhancement. J. Magn. Reson. Imaging, 3: 417-423. doi: 10.1002/jmri.1880030218] Other applications are spin-lock measurements (Ulmer, J. L., Mathews, V. P., Moran, P. R. (1996), Magnetization Transfer or Spin-Lock? An Investigation of Off-Resonance Saturation Pulse Imaging with Varying Frequency Offsets, Journal of Magnetic Resonance, 163: 318-324) and T1_rho weighted imaging (Borthakur, A., Sochor, M., Clark, C. M. (2008), T1ρ MRI of Alzheimer's disease, NeuroImage, 41: 1199-1205), where off-resonant RF is applied, and lock-in separation is used to extract the signal spectrally via demodulation. (Kotler, S., Akerman, N., Keselman, A., Ozeri R. (2011), Single-ion quantum lock-in amplifier, Nature, 473: 61-65. doi: 10. 1038/nature10010] For continuous field monitoring experiments (Sipila, P., Wachutka, G., Wiesinger, F. Coherent excitation scheme to operate pulsed NMR probes for real-time magnetic field monitoring, In: Proceedings, 17th Annual Meeting of ISMRM, 2009), data acquisition should not be interrupted, implying a need for concurrent detection of RF and MRI signal. One of the most significant applications is ultra-short T2 imaging. (Tyler D J, Robson M D, Henkelman R M, Young I R, Bydder G M. Magnetic resonance imaging with ultrashort TE (UTE) pulse sequences: technical considerations. J Magn Reson Imaging 2007; 25:279-289) For the samples having very short coherence time, acquisition with zero-echo time can be very useful to increase SNR. Former methods for concurrent transmission and reception include RF bridges, Lock-in separation, sideband excitation and separation by frequency modulation. Sideband excitation techniques uses off-resonant excitation on the order of a few megahertz and filtering in time domain provides necessary decoupling. (Brunner, D. O., Pavan, M., Dietrich, B., Heller, A., Pruessmann, K. P. Sideband Excitation for Concurrent RF Transmission and Reception, In: Proceedings, 19th Annual Meeting of ISMRM, 2011) However, off resonant excitation increases RF power requirements to achieve the amount of flip angle possible with on resonant excitation. The spins of a sample will experience Bloch-Siegert shift during off-resonant excitation. Besides, hardware to implement sideband excitation is complex and expensive.
In the continuous shift method, decoupling sufficient to achieve the dynamic range is done by using a hybrid isolator for an orthogonally placed transmit receive coil pair, and the MRI signal is extracted from the frequency modulated RF signal using signal processing algorithms. (Idiyatullin, D., Suddarth, S., Corum, C., Adriany, G., Garwood, M. Continuous SWIFT, In: Proceedings, 19th Annual Meeting of ISMRM, 2011) High dynamic range receiver electronics and extremely accurate tuning of isolator are basic hardware requirements of continuous swift method. Post processing of the received signal requires complex matrix operations with the input RF dependent parameters. Orthogonal transmit receive coil pairs are used also in continuous wave NMR. The use of transmit arrays is an alternative decoupling method or an additional procedure to provide extra decoupling which increases MRI signal level with respect to RF signal.